1. Field of the Invention
The present invention relates to an optical disk device for use as an external mass storage unit for a computer or the like.
2. Description of the Prior Art
One conventional optical disk device is shown in FIGS. 9 through 12 of the accompanying drawings.
FIG. 9 shows the conventional optical disk device in exploded perspective. The optical disk device has an optical disk 40 that can be rotated about its own axis by a spindle motor 41 mounted on a support base 46. A laser beam for recording and reading desired data is converged onto a recording layer on the optical disk 40 by a condensing lens 42. The condensing lens 42 is supported by a swing actuator 43 that roughly positions the condensing lens 42 radially with respect to the optical disk 40 for tracking purpose. The swing actuator 43 is mounted on the support base 46 for angular about a swing axis 44 that is positioned radially outwardly of the optical disk 40. An optical head assembly 45, which is also mounted on the base 46, houses a semiconductor laser, optical components such as prisms and mirrors, and sensors for detecting tracking and focusing errors. A laser beam emitted by the semiconductor laser is guided from the optical head assembly 45 to the condensing lens 42 by a rhomboid prism 47. The condensing lens 42 is supported on an assembly of four springs 48 and can be moved toward and away from the optical disk 40 by a focusing actuator 49 composed of a coil for focusing purpose.
FIG. 10 shows the swing actuator 43 in greater detail. The swing actuator 43 has a pair of magnetic yokes 50a, 50b and a pair of permanent magnets 51, 52 disposed vertically between the magnetic yokes 50a, 50b, the permanent magnets 51, 52 being magnetized such that the permanent magnet 51 has an N pole on its upper surface and the permanent magnet 52 has an S pole on its upper surface. Although not shown, the swing actuator 43 also has another pair of permanent magnets disposed between the magnetic yokes 50a, 50b and positioned in diametrically opposite relationship to the permanent magnets 51, 52 across the axis 44. A swing arm 53 made of a nonmagnetic light metal such as aluminum is also disposed between the magnetic yokes a, 50b and supports a pair of coils 54 thereon.
The permanent magnets 51, 52 form a magnetic circuit as follows: A magnetic line 51a of force leaving the N pole of the permanent magnet 51 passes through the magnetic yoke 50b, and then reaches the S pole of the permanent magnet 52 as indicated by the arrow 52a. Then, the magnetic line 52a of force passes through the magnetic yoke 50a and reaches the N pole of the permanent magnet 51 again. When an electric current flows though the coil 54 in direction indicated by the arrow 55, the swing arm 53 is angularly displaced about the axis 44 in the direction indicated by the arrow 56, thereby moving the condensing lens 42 in the radial direction indicated by the arrow T with respect to the optical disk 40. Since the swing arm 53 has a large moment of inertia, however, the swing arm 53 itself is unable to provide a sensitivity that is high enough to follow the high-speed rotation of the optical disk 40. To avoid this drawback, a galvanometer mirror 58 (see FIG. 11) having a sufficiently small moment of inertia is disposed in a path 57 of the laser beam and angularly moved in the direction indicated by the arrow 59 to swing the laser beam from the condensing lens 42 as indicated by the arrows 60 for highly accurate and sensitive tracking control. Stated otherwise, the condensing lens 42 is positioned by the swing arm 53 for rough tracking control, and the galvanometer mirror 58 is angularly moved for highly accurate and sensitive tracking control.
The condensing lens 42 is actuated to focus the laser beam onto the optical disk 40 in the manner described below. As shown in FIG. 10, a lens holder 61 which supports the condensing lens 42, and the focusing actuator 49 has a focusing coil 62 that is supported on the swing arm 53 through the leaf springs 48. The focusing coil 62 is of a U shape straddling a magnetic yoke 63 (see also FIG. 11). The magnetic yoke 63 is integral with a magnetic yoke 64 (see FIG. 10), the magnetic yokes 63, 64 being jointly of a U-shaped cross section. A permanent magnet 65 is fixed to an inner side surface of the magnetic yoke 64 which faces the magnetic yoke 63. The permanent magnet 65 has an N pole on its surface facing the magnetic yoke 63 and an S pole on its surface facing the magnetic yoke 64. When an electric current flows through the focusing coil 62 in the direction indicated by the arrow 66, since a magnetic line 67 of force is directed as shown, a force is imposed on the focusing coil 62 in the direction indicated by the arrow 68 according to the Fleming's left-hand rule. The condensing lens 42 can be moved in the direction indicated by the arrow F by controlling the direction in which the electric current flows through the focusing coil 62.
The conventional optical disk device shown in FIGS. 9 through 11 have suffered from the following problems: Since the swing arm 53 and the galvanometer mirror 58 need to be controlled for tracking control and the condensing lens 42 needs to be controlled for focusing control, the control system of the optical disk device is relatively complex. The large moment of inertia of the swing arm 53 requires a powerful magnetic circuit for angularly moving the swing arm 53 at high speed for tracking control. Therefore, the magnetic circuit consumes a relatively large amount of electric energy. The optical disk device is relatively complex in structure, and cannot be reduced in size and weight.
FIG. 12 fragmentarily shows, in exploded perspective, another conventional condensing lens assembly. A laser beam reflected by a reflecting mirror 69 is converged by a condensing lens 70 that is supported on a cylindrical lens holder 71. The lens holder 71 is axially slidably and angularly movably supported on a support shaft 73 that is vertically fixed to a base 72 parallel to the optical axis of the condensing lens 70. A focusing coil 74 is wound around the lens holder 71. A pair of tracking coils 75 is mounted on the outer circumferential surface of the lens holder 71 in diametrically opposite relationship to each other. A pair of diametrically opposite focusing permanent magnets 76 and a pair of diametrically opposite tracking permanent magnets 77 are disposed on the base 72 for magnetic coaction with the focusing coil 74 and the tracking coils 75, respectively. Each of the tracking coils 75 is spirally wound in a rectangular shape having four sides. When the tracking coils 75 are supplied with an electric current, they develop tracking forces in combination with the tracking permanent magnets 77. According to the Fleming's left-hand rule, only one side (effective side) of each of the rectangular tracking coils 75 which extends parallel to the support shaft 73 is effective for coaction with the tracking forces. However, the electric current that passes through the other side (ineffective side) of each of the rectangular tracking coils 75 flows in a direction opposite to the direction of the electric current through the effective side. When the ineffective sides of the focusing coils 75 are subject to magnetic forces from the tracking permanent magnets 77, they tend to eliminate the tracking forces. More specifically, when the sides of the tracking coils 75 that extend parallel to the support shaft 73 undergo the same magnetic forces, no tracking forces are developed.
In FIG. 12, the angular movement of the lens holder 71 is limited by the layout of tracking permanent magnets 77 and tracking coils 75, and the lens holder 71 and hence the condensing lens 70 cannot be angularly moved beyond 90.degree..